BILLINGS (AP) – Montana's governor called on the Obama administration Friday to strengthen rules that require oil pipelines to be buried just 4 feet beneath major waterways, after two breaches that spilled a combined 93,000 gallons of crude into the Yellowstone River.

River scouring due to flooding or an ice jam is being investigated as a possible cause of a spill last month that dumped about 30,000 gallons into the Yellowstone upstream of Glendive, temporarily contaminating the city's water supply.

In 2011, an Exxon Mobil pipeline broke during flooding near Laurel and fouled dozens of miles of river bank with oil.

Bullock said Friday in a letter to Transportation Secretary Anthony Fox that he wants existing lines surveyed to determine how deeply they're buried. The governor also asked for more federal inspectors in Montana, which has only one overseeing 3,800 miles of pipelines in the state.

"Clearly, more frequent inspections and oversight are needed so as to avoid another major disaster here in Montana," Bullock wrote.

Transportation officials did not immediately respond to requests for comment.

Federal rules that require lines to be buried at least 4 feet beneath riverbeds were last considered for changes after the Yellowstone spill. Government officials determined 4 feet was sufficient in 2013, after a two-year review.

An analysis of past spills found 16 pipeline breaches caused by flooding since 1993. They released a combined 2.4 million gallons of oil, gasoline, propane and other hazardous liquids. Less than 300,000 gallons was recovered, and the spills companies almost $200 million combined in property damages, lost product, cleanup work and other expenses, according to an AP review.

The oil pipeline industry is largely self-regulated, with most inspections conducted by the companies that operate roughly 60,000 miles of crude lines nationwide.

Since the Exxon spill and other recent high-profile pipeline accidents in Michigan and Arkansas, Congress has boosted spending on pipeline safety.

The Department of Transportation plans to hire roughly 100 more inspectors under a $27 million budget increase approved last year. That would supplement the 150 inspectors already with the agency's Office of Pipeline Safety.